Re: Memoize ANTI and SEMI JOIN inner

On Mon, 31 Mar 2025 at 15:33, Alena Rybakina <a.rybakina@postgrespro.ru> wrote:

I believe it's worth asserting that both inner_unique and single_mode are not true at the same time — just as a safety check.

add_paths_to_joinrel() just chooses not to populate inner_unique for
SEMI and ANTI joins because, as of today's master, it's pretty
pointless to determine that because the executor will short-circuit
and skip to the next outer tuple for those join types anyway. I don't
follow why having both these flags set would cause trouble. It seems
perfectly legitimate that add_paths_to_joinrel() could choose to set
the inner_unique flag for these join types, and if it did, the Assert
you're proposing would fail for no good reason.

I tend to agree with you that someone might set this flag to true for these join types in the future.

However, is it necessary to check that extra->inner_unique must be false for SEMI/ANTI joins here, or am I missing something? It looks a little confusing at this point.

if (!extra->inner_unique && (jointype == JOIN_SEMI || jointype == JOIN_ANTI))

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Regards,
Alena Rybakina
Postgres Professional